Medical Definition of rib

: any of the paired curved bony or partly cartilaginous rods that stiffen the lateral walls of the body of most vertebrates and protect the viscera, that occur in mammals exclusively or almost exclusively in the thoracic region, and that in humans normally include 12 pairs of which all are articulated with the spinal column at the dorsal end and the first 10 are connected also at the ventral end with the sternum by costal cartilages — see false rib, floating rib, true rib